DESCRIPTION: The goal of this project is to elucidate the hormonal determinant of gender differences in the fiber phenotypes of the mouse masseter muscle. Th study will determine the effects of sex hormones on the maintenance of massete muscle phenotypes in adult mice. The mouse masseter was chosen for this projec based on the utility of this species for molecular genetics and previous work showing gender differences in fiber type composition. The specific aims include: (1) the determination of whether the sexual dimorphism in mouse masseter is testosterone dependent; and/or (2) estrogen and progesterone dependent. Additionally (3), the project will evaluate the influence of sex hormones on the size and number of masseter motoneurons. Twenty-four adult mal and female mice will be divided into four groups: intact males, intact females castrated males and ovariectomized females. Six weeks following surgery, animals will be euthanized and the masseter muscles harvested for both immunohistochemistry and immunoblotting to determine masseter muscle phenotypes. Size and number of trigeminal motoneurons will be determined by retrograde labelling with fluorogold. It is hoped that the outcome of this wor will be an experimental basis for establishing a mouse model for the hormonal control of masticatory muscle fiber phenotypes.